This invention relates to a temperature-operated fluid friction coupling for intermittently driving a cooling fan of an internal combustion engine.
In a typical design of a temperature-operated fluid friction coupling, a housing is rotatably supported on a drive input shaft adapted to connect with an internal combustion engine, and the interior space of the housing is divided into a reservoir chamber and a torque transmitting chamber by a partition. On the drive input shaft a rotor is fixed so as to form mutually opposite shearing surface gaps between the housing and the rotor. A cooling fan is attached to an outer surface of the housing which comprises a front cover and a rear body. To the outer surface of the front cover is mounted a temperature-sensitive element made of a bimetal which curves in response to a change of ambient temperature. In the partition, a valve opening is provided and a valve lever for opening and closing the valve opening is mounted. The valve lever is initially stressed in the opening direction. Between the bimetal and the valve lever is disposed an actuating pin which transmits a deformation of the bimetal to the valve lever. As the bimetal is deformed by a change of the ambient temperature, the valve lever is moved toward the same direction through the actuating pin, allowing the valve opening to open or close. Then, viscous fluid flows into or out of the shearing surface gaps to effect or break the torque transmitting performance.
The ambient temperature sensed by the bimetal is generally the temperature of air after having passed through a radiator. For example, at a low temperature below 65.degree. C., the bimetal is kept in a relatively flat shape and the valve lever closes the valve opening. In this situation, viscous fluid is raked out by a dam from the torque transmitting chamber to the reservoir, whereby the fluid-friction coupling is kept in OFF condition. Conversely, at a high temperature above 65.degree. C., the bimetal curves toward the outer direction of the housing permitting the free end of the valve lever to separate from a periphery of the valve opening. In this situation in turn, the viscous fluid flows from the reservoir to the torque transmitting chamber, whereby the fluid-friction coupling is turned into ON condition.
Referring to FIG. 6 of the accompanying drawings in which revolution speeds of a fan attached to a coupling are plotted in relation to air temperatures, the performance curves B represent general characteristics of a conventional fluid-friction coupling in the prior art. For example, under the condition that an input speed from an engine is 2000 rpm, if the air temperature is below 65.degree. C. and the coupling is kept in an OFF condition, the revolution speed of the fan is kept at nearly 900 rpm. This is recognized to be a kind of creeping revolution caused by a relatively small transmitting torque produced by a residual viscous fluid within the shearing surface gaps as well as produced by a bearing friction between the housing and the input shaft. Conversely, if the air temperature becomes above 65.degree. C. and the coupling is turned into ON condition, the transmitting torque is increased, thereby casing the revolution speed of the fan to rise to 1800 rpm.
Since the horsepower absorbed by the fan increases in proportion to the third power of the number of revolutions, accompanied by a great consumption of fuel, it is most desirable to keep a fan idling speed during the OFF condition of the coupling as low as possible, from the view points of reducing the fuel consumption and avoiding the generation of noise. In prior designs, however, the fan idling speed should be set about 900 to 1100 rpm due to the following reasons.
In recent years, popular cars are generally equipped with air conditioners. If a switch of the air conditioner is turned to its ON position immediately after the starting of the engine, the temperature of the condenser of the air conditioner will rise quickly. At this moment, if the fan speed is below 500 rpm, the air flow is so small that the temperature as well as the pressure of refrigerant in the condenser rises excessively. Then, a safety plug is melted or a safety valve is released, resulting in need of repair. Therefore, it has been believed difficult to lower the fan idling speed less than the conventional setting of 900 to 1100 rpm. Under the situation, it is also difficult to suppress the noise during the low temperature time immediately after the starting of the engine. Furthermore, during a warming up time in winter morning, although there is no chance to use the air conditioner, the fan speed becomes higher than necessary, resulting in an extension of the warming up time and consumption of fuel.